As the areal density of hard disk drives continues to increase, the design challenges relating to the tunneling magneto-resistive (TMR) film stacks used to manufacture read heads become more acute. One such challenge relates to the stability of the pinned layer (P2) in a TMR transducer.
FIG. 1 illustrates a TMR head film stack 100. The TMR head film stack 100 includes two NiFe outer shield layers S1 and S2. Over a seed layer (SL) of Ta or the like is disposed an antiferromagnetic material (AFM), such as IrMn or PtMn. Over antiferromagnetic material AFM is disposed a synthetic antiferromagnet made up of a ferromagnetic material (e.g., CoFeB) pinning layer P1, a spacer layer of Ru, and a ferromagnetic material (e.g., CoFeB) pinned layer P2. As can be seen with reference to FIG. 1, the magnetic field of the pinning layer P1 is oriented out of the page, and the magnetic field of the pinned layer P2 is oriented into the page. Over the synthetic antiferromagnet SAF is disposed an insulating tunnel barrier of AlO or MgO, over which is disposed a free layer (FL). A magnetic field of the free layer is oriented from right to left. Surrounding the foregoing layers is an insulator such as Al2O3, which serves to separate the foregoing layers from a CoPt hard bias layer (HB), which has a magnetic field oriented from right to left. Disposed over the free layer FL and the hard bias HB is a metal spacer, such as Ru or Ta.
In TMR heads, the pinned layer P2 plays an important role with respect to reliability and performance. The robustness of the head is directly related to both the magnetic coupling strength within the synthetic antiferromagnet (SAF), and the SAF/antiferromagnetic (AFM) integrity. To ensure head robustness, strong couplings are desired between the P1 and P2 layers and for exchange coupling between P1 and AFM. In this regard, low pinning strength can lead to early drive reliability failure. Poor coupling can also deteriorate head performance through pinned layer reversal. Inferior SAF/AFM coupling can cause head baseline noise or baseline popping degradation. Therefore, characterizing the pinning properties in the TMR film is important not only for new product development, but also for a volume production environment.
According to one aspect of the subject disclosure, the coupling strength between the pinning layer P1 and the antiferromagnet AFM may be several thousand Oersted (e.g., 3000 Oe). The coupling and/or pinning properties can be characterized at the slider or Head Gimbal Assembly (HGA) level using a high field quasi-static tester (QST) with a field of around 1.5 T. Applying such a powerful magnetic field to a TMR head, however, can permanently degrade the performance thereof. Testing with a high field QST is therefore usually only used for failure analysis purposes, and not for volume production.